1. Field of the Invention
This invention relates to a non-invasive blood pressure measurment device for measuring blood pressure of a subject in systemic hypotension and relates to a non-invasive blood pressure measurement device for measuring blood pressure of a subject by non-invasive means using a volume-oscillometric method. More particularly, the invention is directed to an improved non-invasive blood pressure measurement device that can correct measurement errors attributable to vibration based on the degree of variation in oxygen saturation measured simultaneously with the blood pressure.
2. Related Art
A photoelectric volume oscillometric method, for example, is known as a conventional method of measuring blood pressure of a subject by non-invasive means.
The photoelectric volume oscillometric method is designed to measure blood pressure by detecting a feeble change in vibration caused by blood pressure when the pressure of a cuff wrapped around a finger is increased or decreased. That is, this method detects a change in the volume of a blood vessel using a light-receiving element for receiving transmitted light from a light-emitting element, and measures blood pressure based on the cuff pressure and the amplitude of a photoelectric plethymograph signal that is an ac component of a light-receiving signal outputted from the light-receiving element. FIG. 7 shows a relationship between photoelectric cuff pressure and plethysmograph obtained when the cuff pressure is linearly increased. In the volume oscillometric method, a mean blood pressure value Pm can be calculated from a cuff pressure reading obtained when the plethysmograph has reached a maximum amplitude L, and a systolic blood pressure can be measured from a cuff pressure reading recorded when the plethysmograph has reached, e.g., 20% of the maximum amplitude L by further increasing the cuff pressure. Also, a diastolic blood pressure Pd can be calculated from a cuff pressure reading at an inflection point of an envelope of the plethysmograph observed in the course of increasing the cuff pressure from 0 mmHg.
A detail of non-invasive blood pressue measurement using such photoelectric volume oscillometric method is described in a periodical, "Japanese Journal of Clinical Monitoring" (Vol. 13, No. 1, 1990, pp. 75-84), published by the Clinical Monitor Society.
A non-invasive blood pressure measurement device that allows both blood pressure measurement by the volume oscillometric method and oxygen saturation measurement to be made simultaneously is proposed in Japanese Patent Unexamined Publication No. 2-305555.
Since this conventional non-invasive blood pressure measurement device based on the volume oscillometric method is designed to measure blood pressure by detecting a photoelectric volume pulse as described above, in the case where vibration is introduced from an external source when a subject is carried in an ambulance or in the case where noise attributable to movement of his body is picked up, measurement errors have resulted unavoidably.
By the way, when the subject is in shock due to ventricular fibrillation or in a low blood pressure state due to severe hemorage, the conventional non-invasive blood pressure measurement devices cannot provide blood pressure measurements in some cases.
To overcome this problem, an attempt has been made to obtain some kind of blood pressure information from the subject in shock or the like. That is, what is proposed is a non-invasive blood pressure measurement capable of determining that the subject is in systemic hypotension, which is less than 60 mmHg (or in pulsational arrest), by checking if a plethysmograph is detected while observing a plethysmograph signal from the light-receiving element under the cuff pressure set to a predetermined value, e.g., to 60 mmHg.
However, the blood pressure measurement under a cuff pressure of a predetermined value such as 60 mmHg permits a determination of the systemic hypotension of a subject around such predetermined blood pressure value. The blood pressure of a subject in shock cannot be measured by this method.
Amid demands for quick emergency medical treatment, it is of importance that blood pressure be measured immediately after a subject in shock or the like has been carried in the ambulance so that emergency medical treatment can be given him thereafter.